a. Field of the Invention
In the transmission of video, the different processes involved in the encoding, transmission and decoding of the video signal usually introduce quality impairments.
The present invention relates to an apparatus and method for perceptual video quality measurement. The quality measurement may be achieved by making a comparison between a reference signal and a degraded signal, or may be based on the degraded signal alone. In particular this invention provides a parameter for use in assessing video quality based on temporal frame freezing.
b. Related Art
Patent Application No WO2006103327 describes a method to measure the quality degradation of a video impaired by frame dropping. A model is proposed for predicting quality decrease due to frame dropping in which the impact of the overall degradation is modeled as an integration (summation) of the individual effects. This model combines the quality function for a single burst of dropped frames, the density of discontinuities for several bursts of dropped frames and a power function varying with the burst density. The overall quality of the processed video is expressed as the quality of the reference video minus the overall contribution of all the temporal frame dropping. The overall contribution in turn is expressed as the sum of individual degradation contribution dt for each burst of duration t. dt is expressed as a power function depending on the distribution of burst duration and the quality function for an isolated burst of dropped frames having duration t. The total duration of the video is not taken into account to calculate the effect of the temporal distortions.
In K. Watanabe, J. Okamoto, T. Kurita, “Objective video quality assessment method for freeze distortion based on freeze aggregation”, in Proceedings of SPIE Conference on Image Quality and Systems Performance III, San Jose, January 2006, vol. 6059 a method is proposed to derive an objective video quality measure from the length of a freezing impairment. In the case where there is only one freezing event occurring in a video, the quality is computed as a logarithmic function of the duration of the freezing event. In the case of several freezing events occurring in the video, the durations of the multiple events are aggregated into one equivalent freeze distortion. Subsequently, an objective video quality measure is computed as a logarithmic function of the equivalent freeze distortion. The method considers the total length of all freeze distortions as the length of the equivalent single freeze distortion. In this method, an individual freeze length is restricted to be a power of 2, e.g. 2, 4, 8, 16, 32, 64.
In K-C. Yang, C. C. Guest, K. El-Maleh, P. K. Das, “Perceptual temporal quality metric for compressed video”, in IEEE Transactions on Multimedia, vol. 9, no. 7, pp. 1528-1535, November 2007 and patent application No WO2007118160A1, a temporal quality metric (PTQM) for evaluating the perceptual impact of frame dropping is proposed. The method maps the number of dropped frames in the video to a local temporal quality metric and uses the variation of this local temporal quality to estimate a global temporal quality for the video sequence. A scene change detector is first applied to detect segments of the video with similar motion activity. A dropping severity estimator s is computed for each frame dropping event in each video segment based on the length of consecutive frames dropped. If no frame is dropped in the segment then s=0, otherwise s increases towards 1 as the length corresponding to the number of consecutive dropped frames increases. Motion activity in each scene is computed based on the average size of valid motion vectors, where valid motion vectors are those with a value above a pre-defined threshold. The motion activity of the scene is used to adjust the dropping severity estimator of each dropping event according to the rule that higher motion activity leads to higher temporal quality degradation. The motion mapped dropping severity for the event is then adjusted in relation to the average motion mapped dropping severity of the 3 preceding frames. If no information exists for previous frames (e.g. beginning of a scene) then a forward scanning window is used instead. A non-linear Temporal Quality Fluctuation (TQF) function is then applied such that the lower the frame rate of the segment of the video, the higher the non-linearity relationship. The TQF is then converted into a temporal quality metric q for each dropping event using a multiplicative transformation. The temporal quality for each scene is computed as the average of the temporal quality corresponding to each dropping event in the scene. The overall temporal quality metric for the video is then computed as the average across scenes.
In patent application WO2007071076A1, a temporal quality analysis is performed as part of a no-reference video quality metric. The temporal quality metric is derived from the percentage of frozen frames in the video, where frozen frames are only considered if not caused by frame rate down-sampling.
The present invention provides an improvement on the above methods. The invention differs from the prior art referred to in the way that the different contributions of individual impairment (freeze) events are accounted for and in the way that a single most perceptually dominant impairment is transformed into a quality metric. Instead of integrating the contributions of all the different impairment events of all different durations, the present invention identifies the impairment duration that contributes mostly to the quality degradation to measure the overall quality degradation and discards impairment events of all other durations. In order to identify which of the impairment durations contributes most to the overall video quality, each individual impairment duration and its total accumulated duration over the video are transformed into a temporal quality metric. Based on the individual values of the temporal quality metric for each impairment duration, one of them is selected. The present invention takes into account both the influence of the individual duration of the impairment and the total accumulated duration of all impairments of that same individual duration, as well as their relative value to the total duration of the video sequence. Furthermore, the present invention also uses the length of the video signal so that the contribution of the degradation is made relative to the total video duration.